Continuous cropping causes impairments such as poor growth, reduction of the yield, and deterioration of quality in many crops. Severe impairments sometimes result in death of plants. These impairments resulting from continuous cropping are attributed mainly to soil-borne diseases.
For example, club roots and the yellows of Chinese cabbages are representatives of the soil-borne diseases of Chinese cabbages. The club roots of Chinese cabbages are caused by Plasmodiophora brassicae. The fungal spores invade into the roots of Chinese cabbages cultivated on the soil, proliferate there, thereby causing the club root disease. As the name of the disease indicates, clubs are formed in the roots of Chinese cabbages suffering from the disease and the clubs inhibit uptake of water and nutrients by the plants from the soil. Eventually, the growth of Chinese cabbages becomes poor, which leads to reduction of their yield and quality. The yellows of Chinese cabbages is caused by Verticillium dahliae, which is a terrestrial fungus. Its spores invade into the roots of Chinese cabbages cultivated on the soil, proliferate there, and cause the yellows. In this disease, Verticillium dahliae inhibits formation of chloroplasts in the crop, generation of carotenoid alone progresses, then the plants yellow. As a result, the growth of Chinese cabbages becomes poor, resulting in reduction of their yield and quality. Moreover, Verticillium dahliae infects flowers and vegetables other than Chinese cabbages. For example, it causes Verticillium wilt in eggplants and tomatoes.
Conventionally, soil has been disinfected generally using chemicals such as PCNB to prevent soil-borne diseases. However, continuous use of soil fumigants subsequent to continuous cropping has facilitated advent of novel pathogens in the soil, which are highly resistant to soil fumigants. Thus, the efficacy of soil fumigants for the pathogen control is reduced. In addition, chemically synthesized soil fumigants may possibly give harmful influences on the environment encompassing consumers, producers, and ecosystem. Under these circumstances, the use of fumigants tends to decrease.
Recently, attempts have been made to apply biological control, which is regarded as a safe method to prevent soil-borne diseases. The biological control has been performed using fungi such as Trichoderma, Fusarium, bacteria such as Pseudomonas and Bacillus, and actinomycetes such as Streptmyces. These microorganisms are formulated into agents and put into practice mainly in United States. Prevention of various soil-borne diseases by utilizing these microorganisms has been studied, but practically usable effect has not been obtained yet.
These microorganisms generally grow and inhabit in the rhizosphere (many microorganisms live within the soil of a few millimeter thick surrounding the surface of roots of plants), and they prevent soil-borne diseases. Since rhizosphere, which is an interface between roots of plants and soil, is a complex and unstable environment, the stable preventive effect is hardly obtained in many cases. It is difficult to constantly obtain a sufficient preventive effect, particularly, in Japan, where a wide variety of microorganisms live in the soil. Prevention of soil-borne diseases targeting rhizosphere has not been widely spread among farmers.